The cause of amyotrophic lateral sclerosis (ALS), a cruel neurodegenerative disease, remains unclear. Trans-activating response region (TAR) DNA-binding protein of 43 kDa (TDP-43) has been suggested to have an important role in ALS pathogenesis. In this thesis, we show that a disease linked mutation in matrin 3 (MATR3), a DNA/RNA-binding protein, corresponds to an increased tendency for TDP-43 to aggregate into large and more numerous cytoplasmic inclusions that are the hallmark of ALS. Immunocytochemistry experiments show that MATR3 colocalizes with TDP-43 in vitro. These experiments also show TDP-43 is a component of both MATR3 granules and stress granules, and that MATR3 inclusions are directly adjacent to stress granules or eIF3α inclusions. We hypothesize that, while not being a part of stress granule complex, MATR3 granules are involved in RNA processing via the stress granule pathway by relaying crucial components such as TDP-43. We have also found that compound 8J is able to disaggregate and relocate TDP-43 and MATR3 positive inclusions in vitro. While the mechanism of action of compound 8J remains unclear, fluorescence activated cell sorting (FACS) experiment showed that there was a significant increase in viability in double wild type (matrin 3 and TDP-43) cells when treated with C8J (p-value <.001), which suggests that the TDP-43 and MATR3 cytoplasmic inclusions that were previously observed have a net cytotoxic effect. Together with the in vitro result on C8J, this result also suggests that C8J enhances the survivability of cells by restoring TDP-43 back to the nucleus. MATR3 biochemistry seems to connect to neurodegenerative diseases in several ways. Identifying the pathological connections between MATR3 and TDP-43 physiology will provide us with a greater understanding of ALS pathology.

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abstract: Exome sequencing was used to identify novel variants linked to amyotrophic lateral sclerosis (ALS), in a family without mutations in genes previously linked to ALS. A F115C mutation in the gene MATR3 was identified, and further examination of other ALS kindreds identified an additional three mutations in MATR3; S85C, P154S and T622A. Matrin 3 is an RNA/DNA binding protein as well as part of the nuclear matrix. Matrin 3 interacts with TDP-43, a protein that is both mutated in some forms of ALS, and found in pathological inclusions in most ALS patients. Matrin 3 pathology, including mislocalization and rare cytoplasmic inclusions, was identified in spinal cord tissue from a patient carrying a mutation in Matrin 3, as well as sporadic ALS patients. In an effort to determine the mechanism of Matrin 3 linked ALS, the protein interactome of wild-type and ALS-linked MATR3 mutations was examined. Immunoprecipitation followed by mass spectrometry experiments were performed using NSC-34 cells expressing human wild-type or mutant Matrin 3. Gene ontology analysis identified a novel role for Matrin 3 in mRNA transport centered on proteins in the TRanscription and EXport (TREX) complex, known to function in mRNA biogenesis and nuclear export. ALS-linked mutations in Matrin 3 led to its re-distribution within the nucleus, decreased co-localization with endogenous Matrin 3 and increased co-localization with specific TREX components. Expression of disease-causing Matrin 3 mutations led to nuclear mRNA export defects of both global mRNA and more specifically the mRNA of TDP-43 and FUS. Our findings identify ALS-causing mutations in the gene MATR3, as well as a potential pathogenic mechanism attributable to MATR3 mutations and further link cellular transport defects to ALS. / Dissertation/Thesis / Doctoral Dissertation Neuroscience 2018